Head mounted displays ("HMD") are becoming increasingly widely used for virtual reality games and passive entertainment, "walk through" computer design and demonstration applications, and real time computer assisted machine control. The increasing use of HMD, and the likelihood of their ever greater application, has caused concern regarding possible health and safety complications of their use, and regarding the related question of operator comfort and efficiency over extended periods of use--regardless of whether or not any such potential discomfort might present a health or safety issue.
A prototype head mounted display using two small cathode ray tube ("CRT") screens viewed through prisms and magnifying optics was introduced in 1965. Known head mounted displays produced since that time have been modifications of this prototype device, having stereoscopic capabilities and relying on an assumption that two-dimensional ("2D") images can provide the user with an illusion of three-dimensional ("3D") space. Recent studies have shown that, while this assumption is not entirely incorrect, 3D space cannot be rendered with integrity from 2D images using conventional methods or means. Particular problems arise when a static stereogram having an illusion of the surfaces of a three dimensional object is replaced with a representation of a 3D world with objects at a full range of disparity increments, which objects and increments the observer must attempt to selectively discern.
A 1993 study at the Edinburgh Virtual Environment Laboratory, University of Edinburgh, has confirmed the effects of stress, blurred vision and nausea resulting from use of a conventional head mounted display in accomplishing a task wherein the wearer must rapidly discern objects both near and far in a virtual reality environment. That study concluded that, although dual/split screen presentations provide disparity cues that specify surfaces in depth, the image can only be seen clearly by accommodating (focusing) to the depth of the virtual screen image. A common focal setting for known head mounted displays is around 50 cm, and the physiology of focal accommodation to such relatively short focal lengths (or, for that matter, to any practical focal length given the other constraints of HMD's such as size, weight, cost, and the like) promotes a corresponding degree of convergence (rotation of the eyes inward). This convergence, coupled with any accompanying "proximal" convergence results in a prolonged vergence effort, which has been shown to be related to visual fatigue. Even if it were practical to do so, an attempt to move the virtual image out toward infinity in an effort to alleviate this problem would only amplify a second problem. That is, both accommodation and disparity give powerful cues for depth, and vergence (convergence and divergence) eye movements are psychologically linked and driven by both sources of information. It had been known previously that users of HMD devices must "learn" to decouple accommodation and convergence, but such decoupling can, at best, be only a transient state since the visuo-motor system will constantly struggle to respond to the differing inputs of blur and disparity. Eventually, a system breakdown is inevitable. In this respect, moving the virtual image toward infinity does not assist, because it merely increases the difference between blur-specified and disparity-specified depth clues when the user is studying near objects.
It would seem to be universally accepted that increased resolution would be a desirable improvement in future HMDs. The above referenced Edinburgh University study has also concluded that a reduction of visual stress should also be attained, and further that provision of a variable focal depth is the means for attaining that objective. The authors of the Edinburgh University study asserted that, "There are two approaches to the problem." The first of the only two approaches which were conceived as a result of this extensive study were either to "paint" the image so that rays specifying a given object require accommodation equivalent to the virtual depth of the object, as by using a very fast oscillating lens system, but a lens system which could adjust at the speed required for this approach is not presently feasible. The second approach suggested was to slave the HMD optics to movement of the user's eyes. A relatively slower lens system could be used for this approach. However, available accurate eye tracking systems are very cumbersome and expensive, whereas available smaller eye tracking systems have very poor stability. In addition, all available eye tracking systems have inevitable transmission delays, and the speed of eye movements can cause any such delay to be a significant problem.
Having concluded that both of the conceivable cures to the problem are not presently technologically feasible, the authors of the Edinburgh study suggested that a partial cure might be effected by setting various focal lengths which a user could manually choose, based upon whether he or she was observing near objects, far objects, or middle distance objects in the environment. An alternate suggested cure would be to forsake the stereoscopic system in favor a monoscopic system. However, the authors acknowledged that many applications require a stereoscopic system, as it is simply impossible to effectively simulate 3D space in a monoscopic system using known methods.
Clearly it would be advantageous to have a head mounted display apparatus which could provide realistically defined 3D images without the adverse side effects discussed above. To the inventor's knowledge, all prior art systems have either caused eye strain or other adverse effects, or else have provided less than ideal depth of field definition.